Addressing recurrent atrial fibrillation

ABSTRACT

An implantable medical device is directed to techniques for applying overdrive pacing to one or both atria following termination of an AF episode, to prevent a recurrent AF episode. The implantable medical device such as a pacemaker applies overdrive pacing according to overdrive pacing parameters, and sets the parameters as a function of the response of the patient to overdrive pacing. The parameters may be adjusted upward or downward, so that overdrive pacing may be applied effectively but not over-applied.

TECHNICAL FIELD

The invention generally relates to implantable medical devices (IMD's)such as pacemakers for detecting and treating tachyarrhythmias, and moreparticularly to techniques employed by IMD's for monitoring and treatingrecurrent atrial fibrillation.

BACKGROUND

Some heart patients experience episodes of atrial tachyarrhythmia,including atrial fibrillation (AF). Although AF episodes may not beimmediately life-threatening, AF episodes may be associated with extremesymptoms, a reduced quality of life, and a reduced cardiac output.

For heart patients having a multi-chamber pacemaker, AF episodes presentan additional problem, in that the pacemaker may coordinate ventricularpacing with atrial activity. When an AF episode begins, it isundesirable for ventricular pacing to be timed according to atrialactivity. Accordingly, some pacemakers are equipped with “modeswitching” capability. The principal purpose of mode switching is toprevent the pacing system from delivering ventricular paces that trackatrial activity when the atrium experiences an episode of atrialtachycardia. When the atrial rate is normal, the pacemaker assumes atracking mode, such as DDD/DDDR, in which ventricular pacing tracksatrial activity. When an AF episode occurs, however, the pacemakerswitches to a non-tracking mode, such as DDIR, and paces the ventricleindependently of atrial activity.

When an AF episode occurs, the pacemaker or another medical device mayapply therapy to terminate the AF episode. Therapy may compriseapplication of a shock or a drug. Another therapy for atrial tachycardiais overdrive pacing, in which the pacemaker paces one or both atria at arate faster than their intrinsic rhythm. Overdrive pacing is ofteneffective in disrupting circus arrhythmia and terminating an AF episode.An AF episode may also terminate spontaneously.

It has been observed that a second AF episode may occur within secondsor minutes after the termination of the first AF episode. Although arecurrent AF episode does not always follow the termination of apreceding AF episode, it has been demonstrated clinically that a patientmay have an increased risk of a recurrent AF episode shortly after an AFepisode has terminated. The phenomenon is called recurrent atrialfibrillation.

SUMMARY

In general, the invention is directed to techniques that apply overdrivepacing to one or both atria following termination of an AF episode, inan effort to prevent a recurrent AF episode. In particular, theinvention is directed to techniques for setting or adjusting overdrivepacing parameters to prevent a recurrent AF episode followingtermination of an AF episode. Overdrive pacing parameters may includethe rate and duration of overdrive pacing. Overdrive pacing parametersmay also include other parameters, such as the duration of the waitingperiod between AF episode termination and the onset of overdrive pacing.

Overdrive pacing therapy, in addition to being effective at terminatingan AF episode, may be effective at preventing a recurrent AF episodefrom developing. It is generally not a good practice to engage inoverdrive pacing for long durations to prevent AF from occurring,however, because long duration overdrive pacing may be unpleasant forthe patient and may drain the power supply for the pacemaker. Similarly,overdrive pacing at a high pacing rate may be unpleasant for thepatient. Accordingly, it is generally better for the patient to applyoverdrive pacing for a duration when the therapy has a good chance ofpreventing a recurrent AF episode, but no longer, and to apply overdrivepacing at a rate that is just high enough to prevent an recurrent AFepisode, but no higher.

The techniques of the invention may include collecting information aboutthe response of the patient to overdrive pacing. The techniques mayfurther comprise setting overdrive pacing parameters as a function ofthe response of the patient. When the techniques are applied by apacemaker, for example, the pacemaker monitors episodes of AF andepisodes of recurrent AF, and evaluates whether the overdrive pacingparameters are effective at preventing episodes of recurrent AF. Thepacemaker may adjust the parameters upward or downward, adapting theoverdrive pacing parameters for the benefit of the patient.

The techniques of the invention may be applied by a pacemaker withmode-switching capability. A pacemaker with mode-switching capabilitymay switch modes following detection of an AF episode or a high atrialrate to pace the ventricles at a rate independent of atrial activity,and may apply therapy such as a shock or overdrive pacing to terminatethe AF episode. Because of the possibility of recurrent atrialfibrillation, it may not be beneficial for the pacemaker to resume atracking mode immediately following termination of an AF episode.

Some of the techniques of the invention, however, may be applied with orwithout a mode-switching operation. In other words, some of thetechniques of the invention may be applied by a pacemaker that isconfigured to detect an AF episode and to detect termination of the AFepisode, whether or not the pacemaker switches modes. The pacemaker mayset or adjust overdrive pacing parameters to prevent a recurrent AFepisode even if the pacemaker has not switched pacing modes.

In one embodiment, the invention is directed to a method comprisingapplying overdrive pacing therapy to an atrium according to an overdrivepacing parameter following an episode of atrial fibrillation, monitoringfor an episode of recurrent atrial fibrillation and setting theoverdrive pacing parameter as a function of the monitoring. Examples ofoverdrive pacing parameters include an overdrive pacing rate, anoverdrive pacing duration, and a duration of a waiting period precedingthe overdrive pacing duration.

Detection of an episode of recurrent atrial fibrillation may affect howthe overdrive pacing parameter is set. A parameter such as the overdrivepacing rate may be increased when an episode of recurrent atrialfibrillation is detected. The failure to detect an episode of recurrentatrial fibrillation, however, may also affect how the overdrive pacingparameter is set. The overdrive pacing rate, for example, may bedecreased when few or no episodes of recurrent atrial fibrillation aredetected.

In some embodiments of the invention, threshold-driven techniques may beemployed to set overdrive pacing parameters. One embodiment is directedto a method comprising monitoring for episodes of atrial fibrillation inan evaluation period and computing a measure of episodes of detectedatrial fibrillation in the evaluation period that are followed bydetected recurrent episodes of atrial fibrillation. The method alsoincludes comparing the measure to a threshold and setting an overdrivepacing parameter as a function of the comparison.

In a further embodiment, the invention is directed to an implantablemedical device comprising at least one electrode disposed proximate toan atrium of a heart, and a processor for controlling a pulse generatorto deliver overdrive pacing therapy to the atrium via the electrodeaccording to an overdrive pacing parameter following an episode ofatrial fibrillation. The processor also monitors for a recurrent episodeof atrial fibrillation, and sets the overdrive pacing parameter as afunction of the monitoring.

In an additional embodiment, the device is directed to a method thatincludes detecting a fibrillation of an atrium of a heart, delivering ashock therapy to the atrium and immediately thereafter applying anoverdrive pacing therapy to the atrium. The method may also includemonitoring for an episode of recurrent atrial fibrillation and settingan overdrive pacing parameter as a function of the monitoring.

The invention may offer many advantages, such as the automaticadaptability of overdrive pacing parameters to the particular needs ofthe patient. The overdrive pacing rate and overdrive pacing duration,for example, may be set to levels that are effective in preventingrecurrent AF episodes. The techniques of the invention also keepoverdrive pacing rate and overdrive pacing duration from becomingunnecessarily high.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an exemplary implantable medical devicethat may practice the invention, with a heart.

FIG. 2 is a block diagram illustrating the constituent components of animplantable medical device such as the implantable medical device inFIG. 1.

FIG. 3 is a timing diagram illustrating some of the overdrive pacingparameters that may be set according to the techniques of the invention.

FIG. 4 is a flow diagram illustrating techniques for setting overdrivepacing parameters.

FIG. 5 is a flow diagram illustrating techniques for setting overdrivepacing parameters using thresholds.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of one embodiment of an implantable medicaldevice (“IMD”) 10 that may practice the techniques of the invention. Forexample, IMD 10 may be configured to apply overdrive pacing to one orboth atria following termination of an AF episode, in an effort toprevent a recurrent AF episode. In particular, IMD 10 may set or adjustoverdrive pacing parameters to prevent a recurrent AF episode followingtermination of an AF episode. IMD 10 is a pacemaker comprising pacingand sensing leads 12 and 14 attached to connector module 16 ofhermetically sealed enclosure 18 and implanted near human or mammalianheart 20. Pacing and sensing leads 12 and 14 sense electrical signalsattendant to the depolarization and repolarization of the heart 20, andfurther provide pacing pulses for causing depolarization of cardiactissue in the vicinity of the distal ends of the leads.

Atrial pacing and sensing lead 12 extends from connector module 16 tothe right atrium 22 of heart 20. Atrial electrodes 24 and 26 aredisposed in right atrium 22 at the distal end of atrial lead 12.Ventricular pacing and sensing lead 14 extends from connector module 16to the right ventricle 26 of heart 20. Ventricular electrodes 28 and 30are disposed in right ventricle 26 at the distal end of ventricular lead14. The electrodes disposed on the distal ends of leads 12 or 14 may beunipolar or bipolar electrodes.

IMD 10 may pace ventricle 26 with electrodes 28 and 30. IMD 10 maycoordinate ventricular pacing with atrial activity sensed via atrialelectrodes 24 and 26. Atrial electrodes 24 and 26 may also be employedto sense an atrial tachyarrhythmia such as AF, and to administertherapy, such as overdrive pacing. IMD 10 may be capable of switchingpacing modes upon detection of atrial tachyarrhythmia via atrialelectrodes 24 and 26.

Overdrive pacing administered via atrial electrodes 24 and 26 mayterminate an AF episode in progress, or may prevent an AF episode fromrecurring. The invention is directed to setting the overdrive pacingparameters that will give IMD 10 a good chance to prevent a recurrent AFepisode with overdrive pacing therapy. In particular, IMD 10 mayregulate the rate and duration of overdrive pacing and may regulateother facets of overdrive pacing as well.

FIG. 2 shows a block diagram illustrating the constituent components ofan exemplary IMD 10 in accordance with one embodiment of the invention,in which IMD 10 is a pacemaker having a microprocessor-basedarchitecture. IMD 10 is programmable and may be programmed with anexternal programming unit (not shown in the figures).

The programmer may provide a series of encoded signals to IMD 10 viawireless telemetry. An input/output circuit 40 may be coupled to anantenna 42 to permit uplink/downlink telemetry through an RF transmitterand receiver telemetry unit 44. In addition to transmitting or receivingprogramming instructions, telemetry unit 44 may transmit or receiveinformation. Transmitted and received information may include, forexample, instructions that cause a processor (such as microprocessor 58,described below) to practice the techniques of the invention.Transmitted and received information may also include default overdrivepacing parameters, or one or more overdrive pacing thresholds asdescribed below, or historical data concerning AF episodes collected byIMD 10. Any of a number of programming and telemetry methodologies maybe employed to transmit information to and receive information from IMD10.

Atrial lead 12 and ventricular lead 14 (not shown in FIG. 2) are coupledto input/output circuit 40. For simplicity, IMD 10 in FIG. 2 is shownwith atrial lead 12 connected thereto, but similar circuitry andconnections not explicitly shown in FIG. 2 may apply to ventricular lead14. Lead 12 is coupled to node 46 in IMD 10 through input capacitor 48.Input/output circuit 40 may also deliver pacing stimuli to the atrium aswill be described in more detail below.

Input/output circuit 40 may further receive input from an activitysensor or accelerometer 50, such as a piezoceramic accelerometer bondedto a hybrid circuit located inside enclosure 18 (shown in FIG. 1).Activity sensor 50 typically (although not necessarily) provides asensor output that varies as a function of a measured parameter relatingto a patient's metabolic requirements.

IMD 10 includes a microcomputer circuit 52. Microcomputer circuit 52stores and executes software-implemented algorithms for detecting andresponding to arrhythmias such as AF. In some embodiments of theinvention, IMD 10 may be programmed to operate in variousrate-responsive or non-rate-responsive modes. In addition, microcomputercircuit 52 may store and execute software-implemented algorithms formanaging overdrive pacing parameters according to the techniques of theinvention, and for controlling delivery of overdrive pacing therapyaccording to the overdrive pacing parameters.

Microcomputer circuit 52 may include an on-board circuit 54 andoff-board circuit 56. On-board circuit 54 includes microprocessor 58,system clock circuit 60 and on-board random-access memory (RAM) 62 andread-only memory (ROM) 64. Off-board circuit 56 comprises a RAM/ROMunit. On-board circuit 54 and off-board circuit 56 are each coupled by adata communication bus 66 to digital controller/timer circuit 68.Microcomputer circuit 52 may comprise a custom integrated circuit deviceaugmented by standard RAM/ROM components. Memory 56, 62 or 64 may storeoverdrive pacing parameters, and may store data pertaining to theevaluation and efficacy of overdrive pacing therapy, as will bedescribed below.

Electrical components shown in FIG. 2 are powered by an implantablebattery power source 70. For the sake of clarity, the coupling ofbattery power source 70 to the various components of IMD 10 is not shownin the FIG. 2. VREF and bias circuit 72 generates stable voltagereference and bias currents for analog circuits included in input/outputcircuit 40. Analog-to-digital converter (ADC) and multiplexer unit 74digitizes analog signals and voltages for digital processing.

Operating commands for controlling the timing of electrical stimulationsdelivered to heart 20 by IMD 10 are coupled from microprocessor 58 viadata bus 66 to digital controller/timer circuit 68, where digital timersand counters establish the various refractory, blanking and other timingwindows used in the detection of cardiac activity and the delivery ofelectrical stimulations.

Sensing circuitry coupled to digital controller/timer circuit 68 detectscardiac activity. Cardiac signals detected via lead 12 are processed bysensing circuitry, which includes sense amplifier 76, peak sense andthreshold measurement unit 78 and comparator/threshold detector 80. Ingeneral, sense amplifier 76, peak sense and threshold measurement unit78 and comparator/threshold detector 80 cooperate to sense theoccurrence and timing of cardiac events such as atrial activations.Sense amplifier 76 amplifies sensed electrical cardiac signals andprovides an amplified signal to peak sense and threshold measurementunit 78, which in turn provides an indication of peak sensed voltagesand measured sense amplifier threshold voltages on multiple conductorsignal path 82 to digital controller/timer circuit 68. An amplifiedsense amplifier signal is also provided to comparator/threshold detector80.

Cardiac signals detected via lead 12 may also be received by electrogram(EGM) amplifier 84. In general, the electrogram signal supplied by EGMamplifier 84 preserves the morphology of the cardiac signal. Digitalcontroller/timer circuit 68 may process the electrogram signal suppliedby EGM amplifier 84, and may transmit the electrogram signal to anexternal programmer for observation and analysis by a physician.

Output pulse generator 86 provides amplified pacing stimuli to heart 20through coupling capacitor 88 in response to a pacing trigger signalprovided by digital controller/timer circuit 68. The conditions thattrigger generation of a pacing trigger signal may vary from patient topatient, and the conditions that may trigger generation of an atrialpacing trigger signal need not be the same as the conditions thattrigger generation of a ventricular pacing trigger signal. In anembodiment of the invention, digital controller/timer circuit 68generates atrial pacing trigger signals that cause overdrive pacing ofthe atrium to terminate an AF episode or to prevent a recurrent AFepisode from occurring.

The invention is not limited to application with IMD 10 as depicted inFIGS. 1 and 2. The techniques of the invention may be practiced by, forexample, single-chamber pacemakers, or triple- or quadruple-chamberpacemakers. The invention may be practiced by devices that provide avariety of pacing, cardioversion and defibrillation therapies.

Devices that perform overdrive pacing of the atrium supply pacingstimuli to the atrium at a rate, called the “overdrive rate.” Theoverdrive rate may be expressed as the number of paces supplied per unittime during overdrive pacing. In addition, devices that performoverdrive pacing of the atrium supply pacing stimuli at the overdriverate for a duration of time, called the “overdrive duration.” Theoverdrive rate and overdrive duration are two significant overdrivepacing parameters, but not the only parameters pertaining to overdrivepacing. Other overdrive pacing parameters will be described below.

FIG. 3 is a timing diagram illustrating several overdrive pacingparameters and management of the parameters according to the invention.The timing diagram illustrates intrinsic atrial beats (represented astriangles) and paced atrial beats (represented as squares) on a graph100. The horizontal axis of graph 100 represents time and the verticalaxis represents the atrial rate in beats (intrinsic or paced) perminute.

The left side of graph 100 shows an AF episode 102, characterized by acollection of rapid intrinsic beats. At some point, denoted by referenceline 104, the AF episode terminates. Termination may have beenspontaneous, or termination may have been in response to therapy such asadministration of a medication, administration of an electricalstimulation, or application of overdrive pacing. Following termination104 of the AF episode, the patient may exhibit a normal sinus rhythm106.

In some embodiments of the invention, a pacemaker detects thetermination of the AF episode in a waiting period 108 before applyingoverdrive pacing therapy. The pacemaker may employ any algorithm forsensing termination of an AF episode. The pacemaker may sense six normalsinus rhythm beats, for example, to confirm that the AF episode hasterminated. In other embodiments of the invention, the pacemaker doesnot employ waiting period 108. The pacemaker may, for example, deliver ashock intended to terminate the AF episode, and begin overdrive pacingimmediately after delivering the shock, without a waiting period 108.

Following termination of an AF episode, the patient is at risk ofexperiencing a recurrent AF episode. The term “recurrent” is used hereinbroadly, and is not limited to a particular time period or number ofbeats. For example, one physician may deem that an atrial fibrillationepisode is “recurrent” when the episode follows within a minute of anearlier terminated episode of atrial fibrillation. Another physician maydeem that an atrial fibrillation episode is “recurrent” when the episodefollows within three hundred beats of an earlier terminated episode. Athird physician may use “recurrent” to refer to an atrial fibrillationepisode that follows within ten minutes or six hundred beats of anearlier terminated episode, whichever is longer. The inventionencompasses all such usages of the word “recurrent.”

When the patient is at risk of experiencing recurrent AF episode, apacemaker may begin overdrive pacing by delivering a paced beat 110prior to a scheduled beat 112, thereby driving the atrium to beat fasterthan the intrinsic rate of the atrium. In a typical application, thepacemaker does not abruptly switch to the overdrive pacing rate. Rather,the pacemaker may ramp up to the overdrive pacing rate in a transitionperiod 114, i.e., incrementally increase the pacing rate up to theoverdrive pacing rate. Transition period 114 is not required, however,and the pacemaker may abruptly begin pacing at the overdrive pacingrate.

The pacemaker paces one or both atria at an overdrive pacing rate 116for an overdrive pacing duration 118. Graph 100 shows an exampleoverdrive pacing rate of about 90 beats (i.e., paces) per minute. At theend of overdrive pacing duration 118, the pacemaker may ramp down fromthe overdrive pacing rate in a transition period 120. The pacemaker maycontinue to monitor for AF in a monitoring period 122 that followsoverdrive pacing. The monitoring period may extend for any period oftime.

Even though a pacemaker may administer overdrive pacing therapyfollowing termination of an AF episode, another AF episode maynevertheless recur. The recurrence of an AF episode during overdrivepacing duration 118 may suggest that the overdrive pacing rate is tooslow. The recurrence of an AF episode during monitoring period 122 maysuggest that overdrive pacing duration 118 is too brief. The inventionis directed to techniques by which a pacemaker adjusts overdrive pacingparameters, such as the overdrive pacing rate and overdrive pacingduration, to prevent early recurrence of AF episodes.

FIG. 4 is flow diagram illustrating techniques used by a pacemaker toset the overdrive pacing rate, or the overdrive pacing duration, orboth, following termination of an AF episode or delivery of AF therapy(130). As used herein, “setting” an overdrive pacing parameter isdefined broadly, and includes selecting a new value for a parameter andadjusting a previous value of the parameter.

After termination of an AF episode (130), the pacemaker may undergo awaiting period before beginning overdrive pacing (134). The waitingperiod (132) is, however, not essential to the invention. In somecircumstances, the pacemaker uses a waiting period to detect that the AFepisode has terminated. In some embodiments of the invention, however,the pacemaker may begin overdrive pacing (134) without waiting fordetection of episode termination. The pacemaker may, for example,deliver shock therapy (130) intended to terminate the AF episode, andbegin overdrive pacing (134) immediately after delivering the therapy.

Overdrive pacing begins at an overdrive pacing rate, and continues foran overdrive pacing duration (134) The pacemaker may ramp the pacingrate up to the overdrive pacing rate as shown in FIG. 3. When thepacemaker senses an AF episode during the overdrive pacing duration(136), the recurrence of AF is evidence that the overdrive pacing rateis too slow.

In some embodiments of the invention, the pacemaker may be programmed torecognize that a single observed AF episode during the overdrive pacingduration warrants an increase in the overdrive pacing rate. In otherembodiments of the invention, however, a pacemaker may be programmed tonot to increase in the overdrive pacing rate based upon a singleepisode. In these embodiments, a single recurrence of AF may be evidencethat the overdrive pacing rate is too slow, but insufficient by itselfto warrant a change to the overdrive pacing rate.

Criteria that warrant an increase in the overdrive pacing rate maydefined in any number of ways. One pacemaker, for example, may beprogrammed to increase the overdrive pacing rate when recurrent AFepisodes have been detected following two consecutive terminated AFepisodes. Another pacemaker may be programmed to increase the overdrivepacing rate using a threshold-driven technique, as will be describedbelow. The invention encompasses all criteria for deciding whether anincrease in the overdrive pacing rate is warranted.

When an increase in the overdrive pacing rate is warranted (138), thepacemaker increases the overdrive pacing rate (140). The amount of theincrease may be a pre-determined value, such as five beats (i.e., paces)per minute. The amount of the increase may also be defined in other waysas well, such as a five percent increase above the current overdrivepacing rate.

The increase in the overdrive pacing rate may take effect at any time.In a typical application of the invention, the increase in the overdrivepacing rate takes effect following termination of the AF episode inprogress. When the AF episode in progress terminates (130), overdrivepacing may begin (134) with the increased overdrive pacing rate.

Upon the expiration of the overdrive pacing duration, overdrive pacingends and a monitoring period begins (142). The pacemaker may ramp thepacing rate down from the overdrive pacing rate as shown in FIG. 3. Ifthe pacemaker has previously switched from a tracking mode to anon-tracking mode, the pacemaker may switch back to a tracking mode uponthe expiration of overdrive pacing duration, or the pacemaker may switchback to a tracking mode at another time.

When an AF episode is detected during the monitoring period (144), therecurrence of AF in the monitoring period is evidence that the overdrivepacing duration is too brief. The pacemaker may determine whether theevidence of an AF episode in the monitoring period warrants an increasein the overdrive pacing duration (146). As with increases to theoverdrive pacing rate, a single observed AF episode during themonitoring period may or may not warrant an increase in the overdrivepacing duration. A pacemaker may be programmed, for example, to increasethe overdrive pacing duration using a threshold-driven technique, aswill be described below, or using other criteria. The inventionencompasses all criteria for deciding whether an increase in theoverdrive pacing duration is warranted.

When an increase in the overdrive pacing duration is warranted (146),the pacemaker increases the overdrive pacing duration (148). The amountof the increase may be a pre-determined value, such as five seconds, ora percentage increase over the current overdrive pacing duration, forexample.

The increase in the overdrive pacing duration may take effect at anytime, but typically the increase in the overdrive pacing duration takeseffect following termination of the recurrent AF episode.

The physician for the patient may set default values for overdrivepacing parameters such as the overdrive pacing rate and the overdrivepacing duration. Following episodes of AF, the pacemaker may adapt theoverdrive pacing parameters so as to address recurrent AF moreeffectively. In the event no AF episodes are detected during theoverdrive pacing duration and during the monitoring period, thenoverdrive pacing may be considered to be successful. When several AFepisodes terminate and few or no recurrent AF episodes take place, thisresult is evidence that the overdrive pacing is effective in preventingrecurrence of AF episodes.

Even if no recurrent AF episodes are detected, however, it is notnecessarily true that the overdrive pacing rate and overdrive pacingduration are best for the patient. It is possible, for example, that thephysician may have set the default values for these parameters higherthan necessary. It is also possible that, even though the overdrivepacing rate and the overdrive pacing duration may have been increasedaccording to the techniques described above, the parameters may havebeen increased by too large an amount, and that lower values for theparameters will be equally effective in preventing recurrent AF. Ingeneral, overdrive pacing should be used when the patient is at risk ofexperiencing a recurrent AF episode, and overdrive pacing should not beused when the risk has passed.

From the standpoint of the patient, it is desirable to use an overdrivepacing rate and an overdrive pacing duration that are high enough toprevent recurrent AF, but no higher. Overdrive pacing can beuncomfortable for the patient, causing symptoms such as loss of sleepand loss of enjoyment of life. In general, the discomfort increases asthe overdrive pacing rate and the overdrive pacing duration increase. AFmay be uncomfortable as well, so it is desirable to strike a balance inwhich overdrive pacing is applied to be effective at preventingrecurrent AF, but is not over-applied. In addition, providing too muchoverdrive pacing causes a drain the power supply for the pacemaker.

Accordingly, the techniques depicted in FIG. 4 also allow the pacemakerto decrease the overdrive pacing rate, the overdrive pacing duration, orboth. The pacemaker may decrease either parameter following themonitoring period (150). The pacemaker determines whether a decrease inthe overdrive pacing rate or overdrive pacing duration is warranted(152). The pacemaker may be programmed to recognize, for example, that acertain number of AF episodes, followed by no recurrent AF episodes inthe overdrive pacing duration, warrants a decrease in the overdrivepacing rate.

When warranted, the pacemaker may decrease the overdrive pacing rate orthe overdrive pacing duration or both (154). The amount of the decreaseneed not be the same as the amount of the increases (140, 148). When alater AF episode terminates (130), the overdrive pacing rate oroverdrive pacing duration will again be evaluated for efficacy, and maybe increased or decreased again. In a typical application, theparameters are set independently. When the pacemaker increases theoverdrive pacing rate, for example, the pacemaker may increase theoverdrive pacing duration, or decrease the overdrive pacing duration, orkeep the overdrive pacing duration unchanged.

In this way, the overdrive pacing rate and the overdrive pacing durationmay be increased or decreased. As a result, the overdrive pacing rateand the overdrive pacing duration may adapt to prevent recurrent AF, andto do so without over-application of overdrive pacing therapy.

In one embodiment of the invention, the pacemaker may increase anddecrease the overdrive pacing rate and the overdrive pacing durationover time, without settling on a final value for either parameter. Inanother embodiment of the invention, however, it may be possible to forthe parameters to settle at stable values.

FIG. 5 is a flow diagram illustrating a threshold-driven technique forsetting overdrive pacing parameters. With this technique, there is alearning period, or “evaluation period,” during which the pacemakercollects data about recurrent AF episodes. During the evaluation period,the pacemaker monitors for episodes of atrial fibrillation and monitorsfor episodes of recurrent atrial fibrillation that follow. After thedata are collected, a measure of recurrent AF episodes per terminated AFepisodes is computed and compared to a high threshold and a lowthreshold, and the overdrive pacing parameter may be set as a functionof the comparison.

FIG. 5 illustrates the technique with an overdrive pacing rate, but asimilar technique may be employed for other overdrive pacing parametersas well. During an evaluation period, the pacemaker collects data (160)pertinent to the overdrive pacing rate. An evaluation period may bespecified, for example, as a number of days, such as fourteen days. Anevaluation period may also be specified as a function of a number ofepisodes, such as a number of AF episodes and recurrent AF episodes.

During the evaluation period, the pacemaker records the number of AFepisodes that have been detected. Following termination of these AFepisodes, the pacemaker may have applied overdrive pacing therapy. Thepacemaker further monitors for recurrent atrial fibrillation episodes.The pacemaker records the number of detected episodes of recurrentatrial fibrillation.

At the expiration of the evaluation period, it is possible that theremay have been too few detected episodes of AF to be statisticallysignificant (162), and in that event, the overdrive pacing rate mayremain unchanged and the pacemaker may reset the evaluation period(164). Data collected during the evaluation period may be discarded(166) or retained. In the example of FIG. 5, the data are retained, anddata collected in the new evaluation period are added to previouslycollected data.

When the pacemaker has collected enough data (162), the pacemakercomputes a measure of the AF episodes in the evaluation period that werefollowed by recurrent episodes (168). The recurrent episodes may haverecurred during the overdrive pacing duration. This computation may beexpressed, for example, as a percentage of recurrent episodes per totalAF episodes (denoted in FIG. 5 as % ERAF).

The measure is compared to a maximum threshold and a minimum threshold.When % ERAF is greater than the maximum threshold (170), then theoverdrive pacing rate may be too low. Accordingly, the pacemakerincreases the overdrive pacing rate (172). Similarly, when % ERAF isbelow than the minimum threshold (174), then the overdrive pacing ratemay be too high, and the pacemaker decreases the overdrive pacing rate(176). The pacemaker may reset data collection (166) and reset theevaluation period (164) and begin collecting new data under the newlyset overdrive pacing parameters.

As with the techniques described above in connection with FIG. 4, theamount of the increase or decrease may be a pre-determined value, or apercentage of the current overdrive pacing rate, or some other increaseor decrease. The amount of a decrease need not be the same as the amountof an increase.

The physician for the patient may set the amount of increase ordecrease, and may set the thresholds that govern whether an increase ordecrease takes place. The physician may also set maximum or minimumvalues for the parameters. For example, the physician may program thepacemaker such that the overdrive pacing rate will never exceed 130beats per minute and will not be lower than 70 beats per minute.

The computation of % ERAF is an illustrative example of a measure thatmay be compared to a maximum threshold and a minimum threshold, but itis not the only possible quantity that may be used. In some embodimentsof the invention, different computations may be performed. The collecteddata may be analyzed by, for example, preparing a histogram or datadistribution as a function of the time elapsed between the terminationof the AF episode and the recurrent AF episode. % ERAF may therefore beembodied as a data distribution, rather than as a single value. Thethresholds likewise may be sets of threshold values, rather than onemaximum or minimum value.

Furthermore, the techniques shown in FIG. 5 are not limited to settingthe overdrive pacing rate. A pacemaker may apply similar data collectionand threshold comparison techniques for setting the overdrive pacingduration. In particular, a pacemaker may collect data during anevaluation period about the number of episodes that occur within amonitoring period following the overdrive pacing duration. The pacemakermay calculate a percentage of recurrent AF episodes that occur in themonitoring period, and may compare the percentage to high and lowthresholds. When the percentage is above the high threshold, thepacemaker may increase the overdrive pacing duration, and when thepercentage is below the low threshold, the pacemaker may decrease theoverdrive pacing duration.

Moreover, the techniques shown in FIGS. 4 and 5 may be applied toparameters other than or in addition to the overdrive pacing rate andthe overdrive pacing duration. Other overdrive pacing parameters mayinclude the duration of the waiting period, for example. Setting thewaiting period may affect how quickly overdrive pacing therapy beginsfollowing termination of an AF episode. The techniques shown in FIGS. 4and 5 may also be applied to discontinue or change the duration of atransition period such as ramping up transition period 114. Theinvention may offer several advantages. A pacemaker may set overdrivepacing parameters that adapt to the particular needs of the patient. Thepacemaker works to provide overdrive pacing at a rate and duration thatare effective in preventing recurrent AF episodes, while keeping therate and duration from becoming unnecessarily high. When a patient needsadjustments to the overdrive pacing parameters, the pacemakerautomatically sets the parameters to effective levels withoutintervention or reprogramming by the physician.

In addition, the pacemaker continues to monitor whether the patientcould be well served by less therapy. By reducing the overdrive pacingrate or the overdrive pacing duration when warranted, the pacemaker mayprovide an effective overdrive pacing therapy that is as effective andmore comfortable for the patient. In addition, the pacemaker may set theoverdrive pacing parameters independently of one another.

Various embodiments of the invention have been described. Theseembodiments are illustrative of the practice of the invention. Variousmodifications may be made without departing from the scope of theclaims. For example, the invention is not limited to a particularphysical construction of a pacemaker or to any particular leadplacement. Nor is the invention limited to use with any specifiedarrhythmia detection or therapy algorithms. Any implantable devicecapable of providing overdrive pacing therapy may practice embodimentsof the invention.

Moreover, the invention encompasses techniques that define overdrivepacing parameters in different ways. For example, the inventionencompasses embodiments in which the “overdrive pacing rate” is definedin terms of an “overdrive pacing interval,” i.e., a time intervalbetween successive paces. The invention encompasses embodiments in whichthe “overdrive pacing duration” is defined to include the time intervalduring which the pacemaker paces an atrium at the overdrive pacing rate,and the invention also encompasses embodiments in which the “overdrivepacing duration” is defined to include one or more transition periods,such as transition periods 114 and 120 in FIG. 3. In addition, FIG. 3depicts transition period 120 as included in monitoring period 122, butthe invention encompasses embodiments that do not include a transitionperiod in the monitoring period.

The invention may be embodied as a device that carries out thetechniques described above. The invention may also be embodied as acomputer-readable medium comprising instructions that cause a processor,such as microprocessor 58 or digital controller/timer circuit 68 shownin FIG. 2, to practice the techniques. A “computer-readable medium”includes but is not limited to read-only memory, Flash memory and amagnetic or optical storage medium. The medium may comprise instructionsfor causing a programmable processor to set one or more overdrive pacingparameters as a function of sensing following an episode of atrialfibrillation. These and other embodiments are within the scope of thefollowing claims.

1. A method comprising: controlling a pulse generator to deliveroverdrive pacing therapy to an atrium via an electrode disposedproximate to the atrium of a heart according to an overdrive pacingparameter following an episode of atrial fibrillation; monitoring for anepisode of recurrent atrial fibrillation; receiving a threshold via atelemetry unit; computing a measure of episodes of atrial fibrillationin an evaluation period that are followed by recurrent episodes ofatrial fibrillation; comparing the measure to the threshold; and settingthe overdrive pacing parameter as a function of the comparing.
 2. Themethod of claim 1, wherein the overdrive pacing parameter comprises atleast one of an overdrive pacing rate, an overdrive pacing duration, anda waiting period preceding the overdrive pacing duration.
 3. The methodof claim 1, further comprising detecting a termination of the firstepisode of atrial fibrillation prior to applying the overdrive pacingtherapy.
 4. The method of claim 3, further comprising waiting for awaiting period between the termination of the first episode of atrialfibrillation and applying the overdrive pacing therapy.
 5. The method ofclaim 1, further comprising: administering a shock therapy to the atriumfollowing a detection of the first episode of atrial fibrillation; andapplying the overdrive pacing therapy immediately after administeringthe shock therapy.
 6. The method of claim 1, further comprisingmonitoring for the occurrence of an episode of recurrent atrialfibrillation while applying the overdrive pacing therapy.
 7. The methodof claim 6, further comprising: applying the overdrive pacing therapy atan overdrive pacing rate; detecting an episode of recurrent atrialfibrillation while applying the overdrive pacing therapy; and increasingthe overdrive pacing rate.
 8. The method of claim 1, further comprisingmonitoring for the occurrence of an episode of recurrent atrialfibrillation in a monitoring period after applying the overdrive pacingtherapy.
 9. The method of claim 8, further comprising: applying theoverdrive pacing therapy for an overdrive pacing duration; detecting anepisode of recurrent atrial fibrillation in the monitoring period; andincreasing the overdrive pacing duration.
 10. The method of claim 1,further comprising: applying the overdrive pacing therapy at anoverdrive pacing rate for an overdrive pacing duration; and decreasingat least one of the overdrive pacing rate and the overdrive pacingduration.
 11. A computer-readable medium comprising instructions forcausing a programmable processor to: control a pulse generator todeliver overdrive pacing therapy to an atrium via an electrode disposedproximate to the atrium of the heart according to an overdrive pacingparameter following a first episode of atrial fibrillation; monitor foran episode of recurrent atrial fibrillation; receive a threshold via atelemetry unit; compute a measure of episodes of atrial fibrillation inan evaluation period that are followed by recurrent episodes of atrialfibrillation; compare the measure to the threshold; and set theoverdrive pacing parameter as a function of the comparing.
 12. Themedium of claim 11, wherein the overdrive pacing parameter comprises atleast one of an overdrive pacing rate, an overdrive pacing duration, anda waiting period preceding the overdrive pacing duration.
 13. The mediumof claim 11, the instructions further causing the processor to detect atermination of the first episode of atrial fibrillation prior toapplying the overdrive pacing therapy.
 14. The medium of claim 13, theinstructions further causing the processor to wait for a waiting periodbetween the termination of the first episode of atrial fibrillation andapplying the overdrive pacing therapy.
 15. The medium of claim 11, theinstructions further causing the processor to: administer a shocktherapy to the atrium following a detection of the first episode ofatrial fibrillation; and apply the overdrive pacing therapy immediatelyafter administering the shock therapy.
 16. The medium of claim 11, theinstructions further causing the processor to monitor for the occurrenceof an episode of recurrent atrial fibrillation while applying theoverdrive pacing therapy.
 17. The medium of claim 16, the instructionsfurther causing the processor to: apply the overdrive pacing therapy atan overdrive pacing rate; detect an episode of recurrent atrialfibrillation while applying the overdrive pacing therapy; and increasethe overdrive pacing rate.
 18. The medium of claim 11, the instructionsfurther causing the processor to monitor for the occurrence of anepisode of recurrent atrial fibrillation in a monitoring period afterapplying the overdrive pacing therapy.
 19. The medium of claim 18, theinstructions further causing the processor to: apply the overdrivepacing therapy for an overdrive pacing duration; detect an episode ofrecurrent atrial fibrillation in the monitoring period; and increase theoverdrive pacing duration.
 20. The medium of claim 11, the instructionsfurther causing the processor to: apply the overdrive pacing therapy atan overdrive pacing rate for an overdrive pacing duration; and decreaseat least one of the overdrive pacing rate and the overdrive pacingduration.
 21. An implantable medical device comprising: at least oneelectrode disposed proximate to an atrium of a heart; a pulse generatorcoupled to the electrode; a processor to control the pulse generator todeliver overdrive pacing therapy to the atrium via the electrodeaccording to an overdrive pacing parameter following an episode ofatrial fibrillation, monitor for an episode of recurrent atrialfibrillation, and set the overdrive pacing parameter as a function ofthe monitoring; and a telemetry unit to receive a threshold, wherein theprocessor computes a measure of episodes of atrial fibrillation in anevaluation period that are followed by recurrent episodes of atrialfibrillation, compares the measure to a threshold and sets an overdrivepacing parameter as a function of the comparison.
 22. The device ofclaim 21, further comprising sensing circuitry to detect the episode ofatrial fibrillation and monitor for the episode of recurrent atrialfibrillation.
 23. The device of claim 21, wherein the device operates ina plurality of pacing modes, and wherein the device selects a pacingmode as a function of a detected episode of atrial fibrillation.